The key IPSec protocols are the AH protocol and the Encapsulating Security Payload (ESP) protocol. As I mentioned earlier, the AH protocol verifies that the header information hasn't been tampered with. The ESP protocol handles the actual encryption and decryption of the data.
IPSec can operate in two modes: transport and tunnel. In the transport mode, only the packet payloads are encrypted, leaving the IP headers in plaintext. In the more secure tunnel mode, both the header and the data are encypted. Tunnel mode requires a dedicated device, typically a VPN concentrator, to decrypt the headers and reroute them.
IPSec supports several different enciphering algorithms. The most commonly used algorithm, Advanced Encryption Standard (AES), is widely acknowledged as one of the strongest algorithms available for data encryption. With a minimum key length of 64 bits, AES is strong enough for almost any commercial application. Some vendors' IPSec implementations use the Data Encryption Standard (DES) or Triple DES (3DES) ciphers. DES, whose 40-bit key has been cracked, is generally considered a weak algorithm for all but the lowest security levels. 3DES fixes DES's problems by using the algorithm three times and providing an effective key length of 168 bits. Note that if your VPN solution supports only one algorithm, any devices you add in the future must use that algorithm as well.
PPTP. A consortium of vendors, including U.S. Robotics, Ascend Communications (now part of Lucent Technologies), 3Com, and Microsoft, developed PPTP. VPN software implementations are more likely than hardware implementations to use PPTP, although some VPN hardware vendors (e.g., Lucent in its MAX and Pipeline communication products and Nortel in its Contivity products) use it. PPTP software implementations can't handle high volumes of traffic, but PPTP hardware implementations can. PPTP 1.2 had major flaws, but version 2.0 fixed most of the problems. However, even this version 2.0 as Microsoft has implemented it is weak cryptographically because it still relies on the user's password to generate keys. In addition, PPTP's design and heavy promotion by a few large vendors such as Microsoft have made it suspect in some quarters.
Probably PPTP's biggest advantage is that it lets you create an easy and inexpensive VPN between two Windows computers (e.g., in a RAS or Routing and Remote Access connection). PPTP also doesn't have the NAT-related problems that I mentioned earlier and works with non-TCP/IP protocols such as IPX. So if you're on a tight budget and you need minimal security, PPTP is certainly better than nothing. But even the budget conscious have other alternatives. Windows XP and Windows 2000 support IPSec natively, and I recommend it over PPTP.
Layer Two Tunneling Protocol. Microsoft uses the PPTP extension Layer Two Tunneling Protocol (L2TP) for authentication, as do some hardware vendors on communications hardware such as routers and firewalls. Again, if you can, choose IPSec because it's more widely deployed and supported.
SSH
Secure Shell (SSH) is a secure version of Telnet that you can use to log on and open a command line on a remote machine. You can also use SSH to establish an encrypted tunnel between two machines, effectively creating a VPN. Different versions of SSH use RSA or Digital Signature Algorithm (DSA) for secure key exchange and 3DES or Blowfish for data encryption. You can use a free program such as Stunnel (http://www.stunnel.org) along with a free version of SSH such as OpenSSH (http://www.openssh.org) to tunnel protocols such as Web and mail protocols through an encrypted SSH tunnel. All you need is a machine at either end running both these programs. SSH and Stunnel are an inexpensive way to implement a VPN, although setting up such a VPN requires a lot of configuration and might not scale to handle a large number of machines. An SSH VPN can, however, make a nice solution for connecting two servers that need to communicate securely, such as a Web server and a back-end database server.
Key Length
The heart of the security a VPN provides is its encryption keys—the unique secret that all your VPN devices share. If the keys are too short, VPN data is susceptible to brute-force cracking. You can often choose the key length to use in your VPN implementation. The longer you make keys, the harder they are to break, but the trade-off is that longer keys also require more processor power for encryption and might slow packet throughput. The minimum recommended key length now is 64 bits (128 bits, if possible) for the symmetric ciphers that encrypt the data and 2048 bits for public key cryptography such as RSA. Modern desktop computers can often crack 40-bit and shorter keys, such as those that DES uses.
Companies deploying VPNs internationally might face some restrictions on key length. Although the government has lifted most restrictions on exporting strong cryptography, you might still need to obtain approval. Check with the US Department of Commerce Bureau of Industry and Security's Commercial Encryption Export Controls (http://www.bxa.doc.gov/encryption) for specific restrictions that might exist for your deployment.
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